The present invention relates to novel pyrazole derivatives and their use as light stabilizers for inhibiting the degradation of various polymers and, more particularly, to compounds which incorporate UV absorbers such as pyrazole derivatives and radical scavengers such as hindered amine light stabilizers (HALS), metal complexes of such compounds, and methods for the synthesis of such metal complexes.
Various industrial applications require the use of polymers which are durable and resistant to ultraviolet light. Examples for such applications can be found in the fields of agriculture, building, manufacture of coatings, machine and aircraft constructions and space exploration.
However, many of the polymers used, such as polyester, polyethylene (LD and HD), polypropylene, polystyrene and polycarbonate are susceptible to photodegradation induced by ultraviolet (UV) light, present in sunlight and various kinds of light sources.
The chemical effects of UV irradiation on such polymers are rooted in the presence of carbonyl groups which may have been formed by various uncontrolled reactions during the synthesis or the processing of the polymer. When these carbonyl groups are exposed to UV-light, they are easily excited to singlet and triplet states which could subsequently initiate some of a number of photochemical reactions in which free radicals are a main product.
Thus, the oxidative degradation of polymers is due to a propagation reaction which involves the formation of free radicals such as alkyl (R.) and peroxy (ROO.) radicals, and compounds such as hydroperoxides (ROOH) which easily decompose to free radicals of the type R., RO., and ROO.. Such radicals may take part in further attacks of additional polymer molecules. These repeated radical attacks may result in scission and crosslinking of polymer chains, while scission may turn a solid polymer into a thin liquid and crosslinking may create a brittle polymer network.
Various attempts have been made to develop light stabilizers which inhibit the photodegradation of polymers. These light stabilizers can be classified according to their stabilization mechanisms into several groups, such as: (a) UV absorbers (UVAs), which absorb the harmful UV light and re-emit it as vibrational (thermal) energy without being themselves destroyed, and without sensitizing the oxydation of the polymer; (b) excited state quenchers, which deactivate the excited states (singlet or triplet) of the polymer, by a mechanism of energy transfer which occur between an excited chromophore in the polymer molecule (donor) and the photostabilizer molecule (acceptor); (c) hydroperoxide (ROOH) decomposers which decompose hydroperoxide molecules to non-radical products such as ions; and (d) radical scavengers, which react with free radicals such as R., RO. and ROO., to form non-radical products such as olefins and ions, or stable radicals such as nitroxyl.
Further attempts have been made to incorporate two or more components acting according to the above stabilization mechanisms, such as a UV absorber and an excited state quencher, a UV absorber and a hydroperoxide decomposer, and a UV absorber and a radical scavenger, since such compounds show a synergistic behavior, in which the UV absorber protects the other component from distruction by UV-light, and said other component inhibit the formation of free radicals and thereby extends the useful life of the UV-absorber (see, for example, J. Pospisil, P. P. Klemchuk, "Oxidation Inhibition in Organic Materials", Vol. 1&2, CRC Press, Bola Raton, 1990; G. Scott, "Atmospheric Oxidation and Antioxidants", Elsevier, A-L-NY-T, Vol 1,2,3, 1993; J. Pospisil, Angew. Macromol. Chem. 216 (1994), pp. 135-146; and F. Gugumas, "Polym. Degrad. and Stab.", 44 (1994), pp.273-297 and references cited therein).
Considerable efforts have been made to develop pyrazole-containing dyes, since these compounds were found to have high light stability, mainly due to their effective UV absorbance (See, for example, B. I. Stepanov, Introduction to the Chemistry and Technology of Organic Dyes, publ. "Chemistry", Moscow, 1977, p. 253-268).
Examples of light stabilizers based on pyrazole derivatives and their metal salts or metal complexes are disclosed in Ger. Pat. No. 2,433,111; 2,410,370; 2,519,594; 2,651,272; 2,529,052; and 2,511,354, Fr. Pat. No. 2,777,826, U.S. Pat. No. 4,008,200, Swiss Pat. No. 605,796, Belg. Pat. No. 891,814, and S.U. Pat. Nos. 956,504; 737,401; 2,829,024; 1,427,802; 1,015,664; 1,205,543; 1,005,448; 717,043; and 786,317.
However, most of these light stabilizers do not exhibit efficient radical scavenging activity, and they lose their effectiveness under condiotions of continuous and intensive irradiation.
There is thus a widely recognized need for, and it would be highly advantageous to have, a light stabilizer which incorporates the effective UV absorption of pyrazole derivatives and the efficient radical scavenging activity of hindered amine light stabilizers (HALS) such as piperidine derivatives, and which retains its effectiveness under conditions of continuous and intensive irradiation.